Women have a 2-3-fold greater risk of developing Alzheimer's disease (AD) than men. There is evidence that estrogen replacement therapy reduces this risk. However, no biological mechanism is proven for this protective effect. In AD, acetylcholine-containing neurons in the basal forebrain degenerate. Thus, it is hypothesized that one mechanism for estrogen's effect is to stimulate basal forebrain cholinergic neurons. More specifically, estrogen stimulates the activity of the high affinity choline transporter, which carries out the critical step in the synthesis of acetylcholine. To test this hypothesis, primary neuronal cultures will be prepared from basal forebrain of embryonic rats. These cultures will be enriched for cholinergic neurons and conditions optimized for their surivival. Cultures will be tested for high affinity choline transporter activity. Cells will be treated with estrogen and the effect of this treatment on the activity of the high affinity choline transporter will be measured. High affinity choline transport will be correlated with the number of cholinergic neurons and other markers of cholinergic function including acetylcholine content, choline acetyltransferase activity and high affinity choline transporter mRNA level. The hypothesis will be proven if estrogen increases the activity of the transporter. It is also expected that estrogen will enhance the survival of these neurons in culture. These experiments will be extended to address the effects that phytoestrogens, plant-derived estrogen-like compounds, have on cholinergic survival and function. Identifying the mechanism(s) by which estrogen is protective in AD is important for several reasons. First, it provides a biological rationale for estrogen replacement therapy in postmenopausal women. Second, it provides insights into the pathophysiological processes underlying AD and therefore identifies new targets for the treatment of AD. Lastly, knowledge of estrogen's mechanism of action allows design of drugs that mimic its therapeutic effects, but lack its undesirable side effects.